I have used RTH in sport mode several times as it does indeed return much faster, just never tried to cancel it by switching back into normal mode. try not to set your RTH height much higher than the tallest obstacle in your flight area as this will burn more battery power to attain that altitude prior to returning, and then yet more descending from that height. The tallest obstacle in my area is 90', so my RTH height is 120', an initial battery warning of 25% is plenty, remember that battery warning is dynamic, in that it will alert you when you get to a level that will leave 5% on a RTH. I regularly fly my batteries down to 10-15% prior to landing, its kind of like scuba diving, you want to end up low on air close to the surface, same with the battery, low power close to your home point. It works for me as in 4 years of flying quads have yet to lose one to battery issues, or anything else for that matter.